Our objectives are threefold: a) biology of the rat B lymphocyte, with particular attention to subsets of B cells characterized by density of surface membrane Ig, and attention to the lateral mobility of fast and slow capping surface membrane molecules, the cytoskeleton, and membrane fluidity; b) antigen recognition and antibody formation in B cells; c) biology of B cells in young, aged, and tumor-bearing rats. We shall prepare subsets of B cells according to density of membrane SIg by means of a cell sorter, and examine the lateral mobility of 4 molecules on these subsets: Ig, Fc receptors (fast cappers), AgB and Ia (slower cappers). The cytoskeleton and membrane microviscosity of these B cell subsets will be examined by fluorescence and electron microscopy and by fluorescence polarization and photobleach recovery. The relationship of the cytoskeleton and membrane fluidity to capping will be examined. The effect of cytochalasins, colchicine, Ca2+ and anesthetics on capping cytoskeleton, and membrane microviscosity will be examined; and the effect of antigens and mitogens on these entities will also be studied. Functional assays of B cell subsets, taken from young, aged, and tumor-bearing rats, will include response to mitogens, to T-dependent and T-independent antigens, Ig/Ab production and cell-cooperation.